Switch with swaged leaf-spring contact

ABSTRACT

This invention relates to a switch device having a flexible arm and at least one co-operating contact. More particularly, the invention relates to the modification of the movable arm by swaging, and the method for accomplishing the same, so as to give improved spring characteristics to the movable arm, including reduced metal fatigue.

United States Patent [191 Hammell et al.

SWITCH WITH SWAGED LEAF -SPRING CONTACT Inventors: Kemper MartelHammell; Norwood Claude Graefi, both of Harrisburg, Pa.

Assignee: AMP Incorporated, Harrisburg, Pa.

Filed: Apr. 11, 1973 Appl. No.: 350,298

us. CL, .,....200 246, 200/283, 2 9/630 c, 29/1310. 1:; 1m. (:1. 1101111/26 Field of Search 200/166 J, 1 A, 166 C;

29/630 C, 630 G, DIG. 18, 622

References Cited UNITED STATES PATENTS 10/1962 Dal Bianco et a]."335/154 [451 Apr. 30, 1974 3,258,557 6/1966 Scheepstra et al 335/1543,015,876 1/1962 Hutt 29/630 C 3,314,028 4/1967 Wyland et a1 200/166 c xPrimary ExaminerRobert K. Schaefer Assistant Examiner-William J. SmithAttorney, Agent, or Firm-Allan B. Osborne [57] ABSTRACT This inventionrelates to a switch device having a flexible arm and at least oneco-operating contact. More particularly, the invention relates to themodification. of the movable arm by swaging, and the method foraccomplishing the same, so as to give improved spring characteristics tothe movable arm, including reduced metal fatigue.

18 Claims, 8 Drawing Figures PATENTEDAPRBO I914 51808390 SHEET 2 [IF 4 Pnmao an (808,390

sum 3 or a Q' COMPRESSIVE TEYNSILE RESIDUAL STR ES -COMPRESSIVE IBENDING STRESS COMPRESSIVE TENSILE 'i' RESIDUAL PLUS BENDING STRESSPATENTEDAPR 30 mm sum u UF 4 TJMILH.VII..U 2

SWITCH WITH SWAGED LEAF-SPRING CONTACT BACKGROUND OF THE INVENTIONMoving-arm electrical switches formed from spring tempered conductivesheet metal are notoriously old in the electrical arts. However suchswitch arms have always been subject to metal fatigue, which has becomean increasingly important factor as the trend to miniaturizationprogressed due the the premium placed on space, weight, and costrequirements by the requirements of the space exploration and thecomputer age.

A correlary to the foregoing is the object of reducing the complexityand number of parts of the electrical device and of its assemblyprocedures.

In addition, in another aspect of this invention, it is an object toform all, or a major portion, of the metal components of the electricalswitch device from the stamping and forming of a uniform strip ofconductive metal, even though the metal characteristics required forvarious metallic parts of the device differ.

These and other objects and advantages have been realized by the presentinvention which contemplates swaging the movable contact arm to give athinned zone across the arm sufficient to concentrate the major portionof the flexing of the arm along the zone. Applicant has discovered thatthis swaging rather than weakening the contact arm (such as would occurif the arm were just nicked); on the contrary increases its longevity bydecreasing the metal fatigue which results from repeated cycling ofthe'arm in making and breaking electrical contact.

Having achieved this improvement, applicants now theorize (withoutwishing necessarily to be restricted to this theory), that the decreasein metal fatigue is due to the introduction of residual compressivestresses in the underlying areas immediately adjacent to the swagedsurfaces (which include the areas of said swaged surfaces). Apparently acantilever beam fixed at one end can better withstand in said outerareas compressive stresses than tensile stresses. The effect of thisprecompression stressing will bediscussed further below with regard tothe drawings.

Various practical applications of this invention have been disclosed andclaimed in related copending applications of ourselves and ourco-worke-rs: Ser. Nos. 306,112; 306,113; and 306,114; all filed on Nov.13, 1972, (the disclosure of which is incorporated herein by reference).

It has been known in the past to deform movable contact arms inelectrical switches. However, these apparently were restricted to snapaction devices where the deforming was restricted to corregating orthinning only a portion of the contact arm blade, often accompanied byaperturing the blade in the vicinity of the localized deformation.Examples of this prior art are to be found in U.S. Pat. Nos. 2,361,202;2,604,316; and 2,814,685. The purpose and structure of these foregoingdevices are clearly differentiable from the present invention asdisclosed and claimed herein.

In this specification and the accompanying drawings applicant have shownand described preferred embodiments of their invention and havesuggested various alternatives and modifications thereof; but it is tobe understood that these are not intended to be exhaustive and that manyother changes and modifications can be made within the scope of theinvention. These suggestions herein are selected and included forpurposes of illustration in order that others skilled in the art willmore fully understand the invention and the principles thereof and willthus be enabled to modify it and embody it in a variety of forms, eachas may be best suited to the conditions of a particular use.

In the accompanying drawings:

FIG. 1 is an exploded perspective view of a simplified specificapplication of a preferred embodiment of the present invention (moreparticularly described in copending application Ser. No. 306,113);

FIG. 2 is a simplified side elevation of the swaged contact arm and theco-operating swaging dies;

FIG. 3 is a simplified perspective of one of the swaging dies;

FIG. 4 is a theoretical graphic representation of residual stressestypically induced by equal working of opposite surfaces of a beam;

FIG. 5 is similar to FIG. 4 showing the theoretical internal stressesresulting from bending a beam without any prior residual internalstresses.

FIG. 6 is a graphic representation illustrating the addition of theresidual stresses of FIG. 4 to the bending stresses of FIG. 5, showing anet reduction in the amplitude of the more destructive tensile stresses;and

FIGS. 7 and 8 are perspective views illustrating the fabrication fromuniformly thick sheet metal of another electrical device embodying thepresent invention (described in more detail in copending applicationSer. No. 306,112).

The switch device 10 illustrated in F IG. 1 is a simple mechanicalcam-actuated single pole single throw switch having a first contactmember 12 with a fixed contact area 14 and a contact member 16 having amovable contact arm 18. The contact arm 18 has a swaged zone 20. Theupright portions of members 12 and 16 are secured in corresponding slots22 and 24 of the switch base member 26 with switch cam 28 rotatablymounted in base member 26. The cam lobe 30 can be rotated to lift thecam arm 18 away from spring contact with fixed contact 14. In thisparticular embodiment the spring arm 18 has a dimpled portion 32 whichacts as a cam follower.

The thinning of the swaged or compressed zone 20 can be more clearlyseen by reference to FIG. 2. In this preferred embodiment, the swagingdies 34 are of identical shape and strike the contact arm 18 equally onboth sides thus symmetrically swaging the arm 18 and introducingsymmetrically oriented internal residual compressive forces,substantially as envisioned by FIG. 4. The striking surface of the dies34 are polished to give a polished impression to the surface of theswaged zone 20 thereby further reducing the likelihood of surface cracksand thus reducing the susceptibility of the movable contact arm'to metalfatigue. The arrows in FIG. 3 indicate the'direction ofgrinding andpolishing to enchance this characteristic.

FIGS. 7 and 8 particularly illustrate how a number of complexly shaped,densely populated, minute components can be stamped and formed from asingle piece of sheet metal 36 of uniform thickness. The individualcontact members l2, 16, 38, and 40, are initially formed with anexpendable webbing 42 to form a frame It can be appreciated that thethickness of the sheet metal 36 must be reasonably thick to form sturdyposts 46 for plugging into printed circuitry or for wire wrapping andsufficiently strong and ductile to accept right angle bends during theforming operation'without failure; all of which gives a thickness andcharacteristics which form a beam too stiff to make a good contactspring arm 20.

Therefore the swaging of the movable contact arm 20 has the advantage ofgiving the thinness required for good flexibility, enhancing thisflexibility by work hardening, and enhancing the formation of theparticular device illustrated in FIG. 7 by extending the length of thecontact arm 20 so that its free end 48, the contact end, is extendedbetween the cooperating contact areas 14 and 50 of respective first andsecond contact members I2 and 38. In this particular embodiment thesheet metal thickness can be 0.008 inches with a number 4 hardness, isthinned in the swaged area 20 by 50 percent, and thereby worked toapproximately a number 10 hardness. This swaging gives the desiredsofter.

spring enabling the use of physically smaller electromagnets with lesscurrent demands to actuate such springs and yet, with the workhardening, the yield strength increases thus permitting greaterdeflection of the spring without causing it to take a set.

The area, contour, and depth of swaging required will vary according tothe physical characteristics of the metal of the contact arm and itsphysical dimensions. For example swaging beyond a 50 percent reductioncan be used, but should not be so deep as to cause rupturing of themetal. Similarly the transition face 35 relative to the flat face 37 ofthe swaging die can be at more than the illustrated angle of theembodiment shown in FIG. 2, but must not be so abrupt as to fracture themetal or introduce a point of weakness. Thus with the surfaces of thedies 34 substantially flat in their center portions and being slightly.chamfered or angled on their sides as shown; the forming dies havingthis configuration will produce a cross-section in the switch arm 18 asshown in FIG. 2. It is interesting to note that during the swagingoperation, the longitudinal extrusion of the metal from between the diestend to give to the transition area 39 of the area 18 a lesser anglethan that of the die face 35.

Referring to FIG. 7, it will be noted that the free end 48 of thecontact arm 18 is illustrated as having an atrest position in contactagainst the fixed contact area 14 of the first contact member 12. Thiscan be achieved by an uneven swaging causing the free end 48 to tend toassume an angled position with respect to the remainder of the contactmember 16 on the other side of the swaged area 20. More commonly, thispositioning is determined independent of the swaging. The at-restposition could alternatively be free of either fixed contact 12 or 50,or could be up against contact 50. Both of the foregoing can be combinedto give an atrest set against one fixed contact and higher residualinternal compression stresses on the sideof the arm 18 away from whichthe arm 18 is moved to break the atrest contact.

As stated previously, the swaging operation produces residualcompressive stresses on the upper and lower surfaces of the switch armand by virtue of these stresses, the tendency towards fatigue failuresis substantially reduced. FIGS. 4-6 illustrate the manner in which theseresidual compressive stresses on the upper and lower surfaces bringabout this improvement in fatigue life.

At the outset, it should be remembered that fatigue failures in beamsare a result of the repeated compressive and tensile stresses which areimposed when the beam is repeatedly flexed in opposite directions. Thetensile stresses produce extremely high unit stresses at surfaceimperfections such as cracks or other surface flaws. The cracks arepropogated as a result of these high unit stresses and failureeventually results. It follows that any reduction in the magnitude ofthe tensile stresses produced during repeated flexure of a beam inopposite directions will reduce the possibility of fatigue failureand/or improve the fatigue life of the beam.

FIG. 4 diagramat ically illustrates the residual stresses produced inthe swaged zone of the spring arm 48 as a result of the swagingoperation. As shown in FIG. 4, compressive stresses are produced on theupper and lower surfaces and the magnitude of these compressive stressesis reduced as the neutral axis or center of the beam is approached. Infact, low level tensile stresses are present in the center of the switcharm. However, such internal tensile stresses are of relatively littlesignificance, since they are not acting on surface flaws and thereforedo not originate nor aggravate such origins of failure.

FIG. 5 shows the straight line stress distribution induced by bending abeam which has not been swaged in accordance with the instant invention(and therefore is without the residual stresses shown in FIG. 4). Thedotted line in FIG. 5 indicates relatively low level tensile stresses onthe upper side and relatively low level compressive stresses on thelower side, a condition which would exist when the blade or switch armend 48 is in the pre-set condition against the lower contact shown inFIG. 7. If the contact arm 18 is flexed from its normal positionupwardly against the upper fixed contact 50, a high level of tensilestress will be induced on one side of the arm and a high level ofcompressive stresses on the other side as shown by the solid line inFIG. 5. It is, of course, this high level tensile stress which causesfatigue failure.

By addition, it is possible to find the resultant stresses when thenormal bending stresses of FIG. 5 are applied to a switch arm havingresidual stresses as shown in FIG; 4. It will be noticed that theresultant maximum tensile stress on the upper side of the switch arm isconsiderably reduced by virture of the addition of the residualcompressive stress to the applied tensile stress. It will also be notedthat the bending stress on the lower side of the switch arm is increased(by the addition of the residual compressive stress) but failureordinarily does not take place on a compressively stressed side of theswitch arm and these compressive stresses are therefore comparativelyunobjectionable. The reduction of the tensile stress, on the other hand,does produce a significant improvement in fatigue life.

It is emphasized that the stress distribution curves shown in FIGS. 4-6are diagramatic and do not represent observed data. In fact, where theparts of the switch arm are extremely small, it is virtually impossibleto measure the stresses induced by flexure thereof. However, an analysisof data on switch arms in accordance with the invention clearly shows asubstantial improvement in fatigue life.

We claim: v

1. An electrical switch comprising a first contact, a movable contactarm formed of substantially flat conductive metal stock, said arm havinga free end and a fixed end, said free end being positioned to be capableof being moved into electrical contact with at least said first contact,said arm being having been swaged to give a zone of spring qualitytransversely thinned across said arm which zone acts as a pivot area forthe deflection of said arm.

2. A device according to claim 1 comprising a second contact which ispositioned relative said first contact with the free end of said armmovable therebetween, and said two contacts are fixed in position.

3. A device according to claim 2 wherein said arm is elongated andformed from sheet metal having physical characteristics of thickness,hardness and ductility which are unsuited, if unmodified, to form aspring contact arm.

4. A device according to claim 1 wherein said swaged zone is flat andtherefore of uniform thickness and is positioned adjacent said fixedend.

5. A device according to claim 4 wherein said arm has been swaged to adegree which work hardens the metal and sets up compressive stresses inthe underlying areas adjacent the swaged surfaces, which compressivestresses are substantially uniform transversely across the zone and insubstantial excess of any compressive forces in the unswaged stockandwhich have longitudinal components which reduce fatiguefrom repeatedtransverse flexing of said arm by significantly reducing or eliminatingthe tensile stresses in said areas normally appearing upon deflection ofsaid arm in making or breaking electrical connection with said contact.

6. A device according to claim 5 wherein said arm is asymmetricallyswaged more on one surface than on the opposing surface to give apermanent angular set to said arm.

7. A device according to claim 5 wherein the underlying areas, adjacentthe swaged surface which faces away from the only direction in whichsaid arm moves from its normal at-rest position, have highercompensating residual compressive forces than are present in the areasunderlying the opposite swaged surface.

8. A device according to claim 7 wherein the portion of said armadjacent said fixed end is angled in the atrest position relative to theportion of said arm adjacent said free end.

9-. A device according to claim 3 wherein said arm has a normal at-restposition in electrical engagement with one of said-contacts.

10. A device according to claim 3 wherein the swaging of said arm hasextended its length.

11. A device according to claim 1 wherein the thickness of said arm isof the order of magnitude of 0.01 inches.

12. A device according to claim 11 wherein the swaged zone is reduced inthickness by an order of magnitude of about one half the mean thicknessof said arm.'

13. A device according to claim 12 wherein the swaged zone issubstantially flat and of uniform cross-' section with a gradualtransition at either end having an order of magnitude of about 14. Adevice according to claim 12 wherein the conductive metal is a springtempered phospher bronze or berillium copper.

15. An electrical switchcomprising a pair of spacedapart fixed switchcontacts, a sheet metal cantilever switch arm, said switch arm having afree end disposed between said contacts and being selectively engageablewith said contacts, said switch arm having a compressedzone'intermediate the ends thereof which is thinned and formed in such amanner as to limit the majority of the flexing to said zone and tointroduce residual compressive stresses in and under the surfacesthereof, suchthat upon movement of said free end between said contactssaid compressive stresses are increased within tolerable limits on theone side of the zone which faces the direction of movement and saidcompressive stresses are decreased on the other side of said zonewithout the development of the high tensile stresses which typicallylead to early metal fatigue, said compressive stresses beingsubstantially evenly distributed'transversely across the compressed zoneof said switch arm.

16. In a switch or the like having first and second fixed contacts andhaving a movable contact arm, said movable contact arm comprising asheet metal stamped and formed arm having a fixed end and a free end,said arm having a swaged zone adjacent to said fixed end, said zonehaving residual internal compressive stresses which are relatively highadjacent to the surface one side of said swaged zone and are relativelylow adjacent to the other surface side of said swaged zone, a firstportion of said arm between said swaged zone and said fixed end defininga reference plane, a second portion of said arm extendingfrom saidswaged zone to said free end normally extending, with respect to saidreference plane, in an oblique direction divergently away from saidswaged zone on said other side, said free end normally being in contactwith said first fixed contact, said second portion of said arm beingpivotally movable with respect to said swaged zone to a position inwhich said arm extends obliquely divergently away from said swaged zoneon said one side so that said free end is in contact with said secondfixed contact, saidrelative ly high compressive stresses on said oneside being increased and said relatively low compressive stresses beingdecreased during movement of said arm from said first position to saidsecond position.

17. Method of forming an electrical switch having a movable cantilevercontact arm and a corresponding fixed contact by progressive stampingand forming from conductive sheet metal of a substantially uniformthickness and of physical characteristics sufficient to withstand theforming procedure but too thick and without the proper springcharacteristics required for said contact arm, comprising the step ofswaging said arm in a zone extending over an area entirely transverselyacross said arm and to a depth sufficient to thin and work harden saidzone to attain the necessary spring characteristics, to limit themajority of the flexing thereto, and to introduce residual compressivestresses in the general underlying area of the surfaces thereof tooptimize the resistance to metal fatigue in said arm by counteracting atleast a substantial portion of the high tensile forces otherwiseencountered in said areas in switching said arm absent such swaging.

18. Method according to claim 17 wherein said swaging step islongitudinally unrestricted and said arm is elongated by the swaging tooverlap said fixed contact.

1. An electrical switch comprising a first contact, a movable contactarm formed of substantially flat conductive metal stock, said arm havinga free end and a fixed end, said free end being positioned to be capableof being moved into electrical contact with at least said first contact,said arm being having been swaged to give a zone of spring qualitytransversely thinned across said arm which zone acts as a pivot area forthe deflection of said arm.
 2. A device according to claim 1 comprisinga second contact which is positioned relative said first contact withthe free end of said arm movable therebetween, and said two contacts arefixed in position.
 3. A device according to claim 2 wherein said arm iselongated and formed from sheet metal having physical characteristics ofthickness, hardness and ductility which are unsuited, if unmodified, toform a spring contact arm.
 4. A device according to claim 1 wherein saidswaged zone is flat and therefore of uniform thickness and is positionedadjacent said fixed end.
 5. A device according to claim 4 wherein saidarm has been swaged to a degree which work hardens the metal and sets upcompressive stresses in the underlying areas adjacent the swagedsurfaces, which compressive stresses are substantially uniformtransversely across the zone and in substantial excess of anycompressive forces in the unswaged stock and which have longitudinalcomponents which reduce fatigue from repeated transverse flexing of saidarm by significantly reducing or eliminating the tensile stresses insaid areas normally appearing upon deflection of said arm in making orbreaking electrical connection with said contact.
 6. A device accordingto claim 5 wherein said arm is asymmetrically swaged more on one surfacethan on the opposing surface to give a permanent angular set to saidarm.
 7. A device according to claim 5 wherein the underlying areas,adjacent the swaged surface which faces away from the only direction inwhich said arm moves from its normal at-rest position, have highercompensating residual compressive forces than are present in the areasunderlying the opposite swaged surface.
 8. A device according to claim 7wherein the portion of said arm adjacent said fixed end is angled in theat-rest position relative to the portion of said arm adjacent said freeend.
 9. A device according to claim 3 wherein said arm has a normalat-rest position in electrical engagement with one of said contacts. 10.A device according to claim 3 wherein the swaging of said arm hasextended its length.
 11. A device according to claim 1 wherein thethickness of said arm is of the order of magnitude of 0.01 inches.
 12. Adevice according to claim 11 wherein the swaged zone is reduced inthickness by an order of magnitude of about one half the mean thicknessof said arm.
 13. A device according to claim 12 wherein the swaged zoneis substantially flat and of uniform cross-section with a gradualtransition at either end having an order of magnitude of about 20*. 14.A device according to claim 12 wherein the conductive metal is a springtempered phospher bronze or berillium copper.
 15. An electrical switchcomprising a pair of spaced-apart fixed switch contacts, a sheet metalcantilever switch arm, said switch arm having a free end disposedbetween said contacts and being selectively engageable with saidcontacts, said switch arm having a compressed zone intermediate the endsthereof which is thinned and formed in such a manner as to limit themajority of the flexing to said zone and to introduce residualcompressive stresses in and under the surfaces thereof, such that uponmovement of said free end between said contacts said compressivestresses are increased within tolerable limits on the one side of thezone which faces the direction of movement and said compressive stressesare decreased on the other side of said zone without the development ofthe high tensile stresses which typically lead to early metal fatigue,said compressive stresses being substantially evenly distributedtransversely across the compressed zone of said switch arm.
 16. In aswitch or the like having first and second fixed contacts and having amovable contact arm, said movable contact arm comprising a sheet metalstamped and formed arm having a fixed end and a free end, said armhaving a swaged zone adjacent to said fixed end, said zone havingresidual internal compressive stresses which are relatively highadjacent to the surface one side of said swaged zone and are relativelylow adjacent to the other surface side of said swaged zone, a firstportion of said arm between said swaged zone and said fixed end defininga reference plane, a second portion of said arm extending from saidswaged zone to said free end normally extending, with respect to saidreference plane, in an oblique direction divergently away from saidswaged zone on said other side, said free end normally being in contactwith said first fixed contact, said second portion of said arm beingpivotally movable with respect to said swaged zone to a position inwhich said arm extends obliquely divergently away from said swaged zoneon said one side so that said free end is in contact with said secondfixed contact, said relatively high compressive stresses on said oneside being increased and said relatively low compressive stresses beingdecreased during movement of said arm from said first positIon to saidsecond position.
 17. Method of forming an electrical switch having amovable cantilever contact arm and a corresponding fixed contact byprogressive stamping and forming from conductive sheet metal of asubstantially uniform thickness and of physical characteristicssufficient to withstand the forming procedure but too thick and withoutthe proper spring characteristics required for said contact arm,comprising the step of swaging said arm in a zone extending over an areaentirely transversely across said arm and to a depth sufficient to thinand work harden said zone to attain the necessary springcharacteristics, to limit the majority of the flexing thereto, and tointroduce residual compressive stresses in the general underlying areaof the surfaces thereof to optimize the resistance to metal fatigue insaid arm by counteracting at least a substantial portion of the hightensile forces otherwise encountered in said areas in switching said armabsent such swaging.
 18. Method according to claim 17 wherein saidswaging step is longitudinally unrestricted and said arm is elongated bythe swaging to overlap said fixed contact.